Embodiments of the present invention relate to electron beam sources that use photocathodes to generate electron beams.
Electron beam sources generate electron beams for applications such as scanning electron microscopy, defect detection, very large scale integration (VLSI) testing and electron beam lithography. In electron beam lithography, for example, a beam of electrons is directed onto an electron sensitive resist layer on a substrate to form a pattern therein. Electron beam lithography is used for making masks for optical lithography, and can also be used to form patterns directly on substrates such as semiconductor wafers and display panels.
In general, electron beam systems include an electron beam source to generate an electron beam and electron optics to shape and direct the beam. For example, one type of electron beam source can generate electrons by exposing a photocathode comprising a photoemissive material to a beam of photons. The incident photons excite electrons present in the photoemissive material, and if the energy transferred from the photons to the electrons is sufficiently greater than the electron emission energy of the photoemissive material, the electrons escape from the photocathode. Electrons escaping from the photocathode can form an electron beam which is then passed through electron optics which can collimate, shape, deflect and modulate the beam.
A single electron beam source can generate a single electron beam, for example, by having a single photon beam incident on a photocathode to generate the electron beam. However, a problem with single electron beam systems is that they have low throughput. To generate a pattern comprising a plurality of pixels, the beam needs to serially expose each pixel in the pattern. Patterns of increasingly higher complexity and resolution involve an increasingly greater number of pixels to be exposed, which can be undesirably time consuming when each pixel is exposed one at a time by the single electron beam.
Systems generating multiple electron beams have a higher throughput than single electron beam systems. One type of multiple electron beam source uses a photocathode that is patterned with electrically controllable electron transmission gates, as, for example, disclosed in U.S. Pat. No. 6,376,985 to Lee et al., which is incorporated herein by reference in its entirety. The gated photocathode of Lee et al. has a substrate on which there is a photoemitter material and a gate electrode. The gate electrode is insulated from the photoemitter material and surrounds an electron emission region of the photocathode. A voltage applied to the gate electrode controls transmission of electrons from the electron emission region. Lee et al. also discloses a gated photocathode having an array of gated emission regions. The gates define a plurality of individual electron beams. The spatial dimensions of the gates control the size and position of each electron beam.
However, gated photocathodes typically contain photoemissive materials that decrease in electron emission efficiency with operation. Thus, the photoyield, or the electron current generated per unit of incident photon power, of the gated photocathode decreases over time. A decreasing photoyield may require an increasing incident photon power, thus undesirably decreasing the energy efficiency of the multiple electron beam source. A decreasing photoyield may also require greater complexity in the control apparatus of the multiple electron beam source. For example, the multiple electron beam source may need a controller to monitor the decreasing photoyield and make necessary adjustments to ensure appropriate pattern exposure.
Another problem with gated photocathodes is that they are susceptible to peformance degradation resulting from nonuniformities or defects that may be present in the photocathode or other components of the multiple electron beam source. For example, the photocathode may contain a photoemissive layer with structural abnormalities, such as, for example, a nonuniform thickness. In another example, a photon source used to generate photon beams incident on the photocathode may generate beams that have a nonuniform intensity or have an intensity variation from beam to beam over the extent of incidence on the photocathode. Both of these examples may result in electron beams of decreased quality and interbeam precision, thus degrading the overall performance of the multiple electron beam source.
Another problem with multiple electron beam sources in general relates to the limits associated with transmission of photons. Photon beams have a transmission resolution limitation related to their wavelength. For example, transmitting photons through an aperture smaller than their wavelength decreases the photon transmission efficiency. However, in some applications it may be advantageous to excite a photocathode with photon beams of increasingly smaller cross-sections. For example, there is a demand in industry to form patterns with ever decreasing pixel size. When the pixel size becomes smaller than this photon wavelength limitation, the performance of the photon portion of a multiple electron beam source using a gated photocathode can be negatively impacted.
Thus, there is a need for a multiple electron beam source that does not have a decreasing usefulness over its lifetime. There is also a need for a multiple electron beam source with increased photon transmission efficiency, stability and uniformity.